1. Field of the Invention
This invention relates to an apparatus and method for screen printing and curing images of multiple colors that includes a shiftable curing device which is swept across a web in close proximity to limiting inked images thereon as the web is held in a stationary position. In preferred embodiments, a screen head which is segmented to permit different colored inks to be applied in separate, segregated areas of the web is movable relative to the web supporting printing table, and the curing device is shifted across the web after the head is raised to cure all of the colored images before the web is advanced one step for subsequent printing and completion of the composite, multiple-colored image.
2. Description of the Prior Art
Conventional apparatus for printing multiple-colored images on a web is normally comprised of a number of separate screen frames or printing heads each adapted to print an image of a different color. Oftentimes, the web is advanced in step-by-step fashion so that predefined areas of the web receive ink from each of the screens until the desired composite, multi-colored image is produced. A registration system electrically interconnected with the web advancement drive is provided to cause predefined areas of the web to be brought into a desired, predetermined positional alignment with each of the printing screens as the web areas are advanced from one screen to the next.
As can be appreciated, it is normally important to insure that the inked image printed by one of the screen printing heads is dried or cured before a second colored image is printed over the first image. Otherwise, interaction between the two differently colored inks may cause the colors to run or bleed, and the sharpness of outline or contour of the composite image will be somewhat diminished. Furthermore, a portion of ink which remains wet on the web may adhere to the screen of the next adjacent, downstream printing unit, thereby causing further interaction of the inks as well as other problems.
In the past, the individual screen printing units of a multiple-colored image printing apparatus have been spaced apart a distance sufficient to allow the wet inks to dry before the images on the web are advanced to the next printing unit. However, in order to reduce the space needed between the separate printing units for applying the necessary colors, stationary heating elements or driers have been disposed between the individual printing units to hasten the curing of the inked images. One example of such construction is illustrated in U.S. Pat. No. 3,223,030 to Ranier, dated Dec. 14, 1965.
In recent years, increased interest has been directed toward inks which are cured by a source of radiation such as ultraviolet, infrared or electron beam. As an example, ultraviolet-curable ink systems offer significant advantages over conventional solvent-based systems because the curing of an ultraviolet-curable coating occurs in a fraction of the time necessary to cure coatings of conventional, heat curable inks. Compact ultraviolet curing devices conserve space in the production line and require a lower quantity of energy as compared to devices for curing an identical image by heat. Moreover, the physical and chemical properties of the ultraviolet-curable film, such as the gloss and chemical resistance of the image, equals and often exceeds in quality the characteristics of good quality conventional inks. In addition, air pollution emissions are reduced since almost all of the liquid coating is converted to solid matter and solvents are not discharged into the atmosphere.
The rate or speed of curing of ultraviolet-curable ink systems is dependent upon the chemical nature of the ink, the thickness and opacity of the coating, the type and color of the printing substrate, and the intensity and proximity of the ultraviolet curing lamp or radiation source. For example, the curing rate of an ink is related to its chemical nature, since the particular monomers of the selected ink will cure at a rate which is dependent on its composition, the nature and amount of sensitizer used, the type and amount of pigment and the quantity and nature of any additives.
The curing rates of ultraviolet ink systems are reduced as the thickness or opacity of the film is increased. The thickness of the ink film, in turn, is dependent upon a number of factors, including the screen mesh count, as the higher mesh counts with smaller openings reduce the amount of ink deposited and thereby decrease the time for curing. The thickness of the ink film is affected by the sharpness and durometer hardness of the squeegee because relatively dull or soft squeegees will result in thicker ink deposits and slower cures. The thickness of the ink deposit is also related to the amount of pressure applied to the squeegee and the angle of inclination of the same. In addition, ink film thickness is related to the thickness of the stencil layer, as heavier ink deposits are observed when the stencil is relatively thick.
The curing time of ultraviolet inks is dependent upon the type of substrate used, as substrates which are white or of a relatively light color exhibit higher reflectivities of ultraviolet light as contrasted to darker substrates. As much as a twenty percent increase in the speed of curing can be expected when images are printed with ultraviolet curable inks on white substrates as compared to comparable images printed upon black substrates.
The nature and disposition of the ultraviolet lamp or radiation source is another factor which affects the curing time for ultraviolet-curable inks. The cure rate is proportional to the square of the ultraviolet light intensity, and curing times can be thus greatly reduced by increasing the power output of the ultraviolet lamp, by focusing the radiation on the surface of the substrate by using reflectors or similar items, or by increasing the number of curing lamps. The length of time that the inked images are subjected to the ultraviolet lamps and the distance between the lamps and the substrate are two other factors which affect the rate of curing.
From the foregoing, it is apparent that a number of factors can significantly influence the curing rate of ultraviolet-curable inks, and therefore it is often a difficult proposition to pinpoint an exact curing speed for any one particular type of ink. Oftentimes, the curing rates are determined experimentally. As a consequence, it is desirable that ultraviolet curing devices be readily adaptable to substantially cure in a minimum amount of time a variety of images formed by different inks, screens or squeegees or applied to any one of a number of different types of substrates.
However, prior art screen printing apparatus having stationary driers interposed between separate screens for printing inks of different colors offer little flexibility insofar as adaption of the curing device to the type of ink, printing unit or substrate is concerned. For the most part, the web is advanced through such apparatus at a rate slow enough to insure that the slowest drying inked image is sufficiently cured.